I would say bleed the brakes as a start just to rule that out.
If the fluid level gets low enough in the reservoir to get air in up there, then you'll essentially need to do a full fluid replacement to get all the air out.
It is possible to get air into the system without loosing fluid by boiling the fluid. Easy to do if you have a stuck caliper and not know it.
While you're bleeding the brakes, you can do a visual on them and see if you need to replace pads/shoes.

Todd

1986 Pickup extracab longbed "Woody edition"
SAS - 33's, 5.29's, lockright rear.

02 IS300 SportCross - my "DD"
94 2wd v6 extracab
04 Outback Wagon- wife's/family car